Sputter-etching, also referred to as backsputtering, has developed recently as an extremely useful technique for the controlled removal of material from surfaces, and particularly for selective removal of material using various masking arrangements. The technique is well known and generally involves a variation of the well-known sputter deposition process using ionized gases excited by a suitable radio frequency or A.C. potential to produce a glow discharge within an evacuated chamber. In sputter-etching, the workpiece from which material is to be removed is the target of the ion bombardment.
The sputter-etching process has inherent advantages for the fabrication of microelectronic elements particularly with respect to the chemical techniques which also are widely used to shape portions of such devices. Sputter-etching generally avoids the undercutting of masks encountered in chemical etching. The sputtering processes are less likely to produce undesirable contamination and the vacuum ambient lends itself to continuous processing including other steps of the fabrication procedure.
However, sputter-etching has a disadvantageous side effect particularly with respect to electronic elements which form the basis of surface-sensitive devices of the field effect type in which electronic control is exercised by electrodes overlaying insulating layers on semiconductor bodies. In particular, the devices of principal interest in this respect are the class known as insulated gate field effect transistors. When devices of this type are subjected to the ion impact process, which is the basis of sputter-etching, secondary electrons are produced at the surface of the electronic element as well as at the cathode surface upon which the elements are mounted. These secondary electrons are accelerated across the full cathode fall of potential localized near the cathode and then drift until they lose their energy through inelastic gas collisions, recombine, or strike the chamber walls or fixtures. In particular, it has been observed that the secondary electrons generated at the cathode strike the anode with a maximum energy corresponding to the maximum cathode to anode potential. Low energy X-rays are thereby produced, some of which, at least, then impinge upon the surface of the electronic element. Even low energy X-rays are able to alter the surface conditions of these elements and thereby affect various parameters of the final devices. In particular, the threshold voltage, which is determinative of the turn-on voltage of an IGFET, is sensitive to X-ray radiation.
Accordingly, it is the object of this invention to suppress such X-rays generated during the sputter-etching process in a diode sputtering system.